Gas turbines are widely used in commercial operations for power generation. FIG. 1 illustrates a typical gas turbine 10 known in the art. As shown in FIG. 1, the gas turbine 10 generally includes a compressor 12 at the front, one or more combustors 14 around the middle, and a turbine 16 at the rear. The compressor 12 and the turbine 16 typically share a common rotor 18.
The compressor 12 includes multiple stages of compressor blades 20 attached to the rotor 18. Ambient air enters an inlet 22 of the compressor 12, and rotation of the compressor blades 20 imparts kinetic energy to the working fluid (air) to bring it to a highly energized state. The working fluid exits the compressor 12 and flows to the combustors 14.
The working fluid mixes with fuel in the combustors 14, and the mixture ignites to generate combustion gases having a high temperature, pressure, and velocity. The combustion gases exit the combustors 14 and flow to the turbine 16 where they expand to produce work.
Compression of the ambient air in the compressor 12 produces an axial force on the rotor 18 in a forward direction, toward the compressor inlet 22. Expansion of the combustion gases in the turbine 16 produces an axial force on the rotor 18 in an aft direction, toward the turbine exhaust 24. A thrust bearing 26 at the front of the gas turbine 10 holds the rotor 18 in place and prevents axial movement of the rotor 18. To reduce the net axial force on the rotor 18, and thus the size and associated cost of the thrust bearing 26, the gas turbine 10 is typically designed so that the axial forces generated by the compressor 12 and the turbine 16 are of comparable magnitude.
FIG. 1 illustrates one design for controlling the net axial rotor force. Air extraction lines 28 connect the compressor 12 to the turbine 16. The air extraction lines 28 provide a pathway for working fluid to bypass the combustors 14 and flow directly to the turbine 16. Separate air extraction lines 28 connect earlier stages of the compressor 12 to later stages of the turbine 16. Through this arrangement, the extracted working fluid has a greater pressure than the combustion gases at the injected turbine stage, thus ensuring that the extracted working fluid travels in the same direction as the combustion gases. The extracted working fluid enters the turbine 16 and joins the flow of combustion gases through the turbine 16, thus increasing the axial force on the rotor 18 in an aft direction, toward the turbine exhaust 24.
The design shown in FIG. 1 has several disadvantages. For example, the extracted working fluid bypasses the combustors 14, thus reducing the volume of combustion gases and overall efficiency and output of the gas turbine 10.
In addition, since the compressor 12 is rotationally coupled to the turbine 16 by the rotor 18, the amount and pressure of the extracted working fluid available is directly dependent on the operating level of the gas turbine 10. While acceptable during steady state operations, this design is less than ideal during partial load operations or transients when the compressor 12 operating level, and thus compressor axial thrust, may be substantially different from the turbine 16 operating level, creating an imbalance in axial forces on the thrust bearing 26 that can result in vibration and instabilities. As a result, the thrust bearing 26 must be larger to accommodate a greater variance in the net axial rotor force during transient conditions or at various operating levels. Moreover, as the amount and pressure of extracted working fluid varies directly according to the operating level of the gas turbine, the design geometry of the turbine rotor is necessarily constrained to produce a desired axial thrust at any given operating level.
Therefore, the need exists for a system and method to control the axial rotor forces independent of the operating levels of either the compressor or the turbine. Ideally, the system and method will minimize the net axial thrust on the thrust bearing during both steady state and transient operating levels, will not reduce the overall efficiency of the gas turbine, and will accommodate optimum rotor geometry to reduce part weight and cost.